If any of these last for more than 3 weeks, you need to see a doctor for sure.
Check for

Any white or red patch on the lining of the mouth.

Any unusual swelling of the jaw or neck,especially if it is stone hard.

Any unusual bleeding from any oral site.

Lung Cancer

Symptoms to watch out for

Persistent difficulty in breathing, wheezing, chest pain or cough or blood in the sputum.

Chest pain that worsens on taking a deep breath or travels from the shoulder to the arm.

If any of the above is present, especially with unexplained loss of weight or fatigue, it is time to see a doctor as soon as possible. Unfortunately, there are no symptoms in the majority of patients at an early stage and symptoms typically develop at a later stage.

Check for
There are no specific things you can look out for yourself, but your doctor may begin with a chest X-ray or CT scan, which may prove to be revealing.

Prostate Cancer

Symptoms to watch out for (symptoms usually manifest themselves when the urethra is pinched by the prostate mass):

Frequent urination with a sense of urgency (which wakes you up too often at night, is difficult to start, stops midstream, and is difficult to control).

Blood in the urine or semen.

Any of these symptoms should prompt a visit to the doctor, especially if you are above 40 years old or have a family history of prostate cancer.

Cervical Cancer

Symptoms to watch out for

Abnormal vaginal bleeding that occurs between periods, or after menopause, or after sexual intercourse.

Persistent pelvic pain or pain after intercourse.

Abnormal mucus-like vaginal discharge.

Cervical cancer is the most common cancer among women in India. While there are no specific things that you can look out for, there is a program of Cervical Cancer Screening available which can detect the disease at an early stage.

All otherwise healthy ladies should discuss the prospect of undergoing this screening with their doctors starting at age 21 years or within 3 years of starting vaginal intercourse (whichever is earlier).